1. Field of the Invention
The present invention relates to Bragg cells for deflecting optical beams at extremely high frequencies, and particularly to Bragg cells which may be used as optical taps in fiber-optical systems.
2. The Prior Art
Bragg cells are well-known for use in deflecting optical beams by interaction of the beams with acoustic waves. Conventional Bragg cells make use of a wide optical beam whose width encompasses many optical wavelengths, and which interacts with an acoustic wave traveling in a solid or liquid medium. An optical beam encountering the acoustic wave at an angle which falls within a given range will be diffracted at an angle which is a function of the acoustic wavelength and the optical wavelength. Typically, these Bragg cells are used as UHF frequencies in the range of about 10 MHz to 2 GHz, with the deflection angle of the light being very small. It is very difficult to make such a device for use at extremely high frequency ranges on the order of several GHz and with larger deflection angles, because of acoustic losses experienced at those frequencies in the solid or liquid medium.
In conventional Bragg cells, the angle of deflection of optical beams varies almost linearly with the frequency of the acoustic wave. This implies that the light pattern formed by the deflected beam is the Fourier transform of the acoustic beam modulation. The availability of this type of interaction opens up a wide range of signal processing applications for these devices. Further, if an optical lens is utilized to perform Fourier transforms, or if interactions with two acoustic beams are utilized, further possibilities for signal processing with acoustic waves in an acousto-optic system are opened up, including applications to real-time wide-band convolution and correlation of electrical signals. However, because of the frequency restriction experienced by these devices, they have not been available for many high-speed uses, such as communications applications and high-speed switching of optical signals in fiber-optic systems. Nevertheless, the use of wide optical beams in these devices precludes their use in many optical systems, such as those utilizing optical fibers as waveguides for carrying narrow band optical signals. One particular application for a Bragg cell having the capability to deflect light at larger angles and at much higher frequencies than conventional Braggs cells, would be its use for tapping optical fibers and optical waveguides.
In many systems involving optical waveguides and optical fibers, it is necessary for system operation to tap light from the fiber or waveguide. Applications for such systems include, among many others, tapping of the light for signal processing purposes such as multiplexing or data distribution, tapping of the light for filtering purposes, and tapping for purposes of monitoring system operation. Many devices and processes are presently utilized for tapping light from optical waveguides as well as for injecting light into the optical waveguides.
One conventional method for tapping light from such a system involves use of optical couplers which are constructed to hold optical fibers in parallel proximity, forming an interaction region between the parallel fibers in which optical signals may be coupled from one fiber to the other. Use of these types of couplers requires careful alignment of the adjacent fibers, and may also require a lapping or other process for removing cladding from around the core portions of each fiber so that the cores may be placed in closer proximity to one another. The amount of light which is tapped from the waveguide is directly related to the relative position of the adjacent fibers. Thus, to change the amount of light coupled from a particular fiber, careful and often difficult and time consuming adjustment of the relative positions of the adjacent fibers in the coupler is necessary. This type of coupling arrangement does ot lend itself to applications where rapid on and off switching is necessary or where changes in amount of light tapped from a waveguide is required.
Another means for tapping light from optical waveguides such as optical fibers involves bending the waveguide at the tap location by an amount sufficient to cause a portion of the light traveling in the waveguide to be emitted from the core as it passes through the bend. This method permits adjustment of the amount of light tapped from the waveguide by adjusting the amount of bend in the waveguide. As the amount of bend increases over a small area, more light will be emitted, and vice-versa. However, like the lapped-fiber configuration described previously, this method does not lend itself for use in providing rapid on and off switching or changes in the amount of light tapped from the waveguide. In fact, such rapid physical adjustment of the bend to accomplish these purposes would not be possible with present fiber handling technology. Further, making changes in the bending radius is an undesirable way of accomplishing switching functions, since the continued adjustment of the bending radius causes fatigue in the waveguide, thereby significantly reducing the operating life of that waveguide.
Another device for tapping light from optical waveguides utilizes an acoustic transducer for sending an acoustic signal through a quartz block at an angle with respect to an optical fiber embraced by the quartz block. The gap between the quartz block and the optical fiber is filled with a liquid to obtain acoustic impedance matching. Light in the fiber is reflected out of the fiber under Bragg's condition through another block positioned below the fiber. This system avoids much physical damage to the fiber, but the use of the block and index matching fluid produce attenuation of the acoustic signal, limiting the acoustic signal frequency which can be used. Further, since the acoustic signal must intersect the small fiber, proper alignment of the acoustic transducer and the optical fiber through the block is critical. However, since the transducer is fixed onto the block which is fixed relative to the fiber, alignment of the device is very difficult during assembly, and changing the alignment after assembly is virtually impossible. In addition, the use of liquid to interface the block with the fiber further complicates the system, as well as its assembly. Thus, like the devices discussed above, this system is limited in the acoustic frequency which can be used, as well as in the assembly and operation of this complex system.
Based on the above, it would be an important improvement in the art to provide a Bragg cell which could be utilized at frequencies which are much higher than conventional Bragg cells, and which would utilize acoustic signals at those frequencies for deflecting light beams in optical waveguides at a relatively large angle with respect to the direction of travel of the original beam. It would be a further improvement in the art to provide such a Bragg cell which could be used for tapping light from optical waveguides, including optical fibers, so that light traveling in the optical fiber could be interrogated, and so that the amount of light going into or out of the waveguide could be controlled and rapidly changed with the assistance of a high-speed external electronic signal. It would be an even further improvement in the art to provide such a device for tapping optical waveguides which provides a solid acoustic path which is liquid-free and which may be initially installed and adjusted, with no further adjustment required in accomplishing the above-described tapping and switching functions. Such a device preferably would operate at acoustic frequencies above approximately 1 GHz and would utilize bulk acoustic waves. It would be a still further improvement in the art to provide an acousto-optic Bragg cell for applying an acoustic signal to an optical signal traveling in a waveguide, with the Bragg cell including means for detecting when the source of the acoustic signal is in position to communicate the acoustic signal into the waveguide at an appropriate angle. It would be an even further improvement in the art to provide an acousto-optic Bragg cell having the source of acoustic signals positioned directly upon a waveguide so that acoustic signals may be applied directly into the waveguide to interact with optical signals traveling therein.